The application of nanosecond laser pulses in ablation-based laser surgery continues to increase owing to the high ablation precision and to the reduction in collateral tissue damage. This, however, has not been accompanied by a clear understanding of the fundamental mechanism. Three major models have been used to explain the soft tissue ablation by nanosecond laser pulses in a spectral region from far ultraviolet to near infrared. In addition to the mutual incompatibility between these models, three fundamental questions remain to be answered: (1) What is the relation between tissue absorption and the ablation threshold? (2) Does plasma occur when a strongly absorbing tissue is ablated near the threshold? (3) What is the role of nonlinear absorption by water in soft tissue ablation? The long term objectives of this proposal are two-fold: to achieve a clear understanding of the fundamental mechanism underlying soft tissue ablation by nanosecond laser pulses; and to provide practical guidelines for substantial improvements in the surgical laser design. We will investigate three hypotheses regarding the soft tissue ablation by nanosecond laser pulses in the spectral region from far UV to near-infrared: (1) the influence threshold depends primarily on the tissue absorption; (2) the nonlinear absorption of laser pulse energy by the water component in soft tissue is significant in tissue absorption near and below the ablation threshold; (3) collateral tissue damage in ablation-based treatment of lesions inside skin tissue can be substantially reduced by optimization of laser and other parameters. This proposal has the following specific aims: (1) We will measure ablation curves from the surface ablation of porcine skin and cornea samples; (2) we will measure the threshold of optical breakdown in water and investigate the transient optical absorption in water; (3) we will develop an experimental device for transient conductance measurement in water; (4) we will investigate the propagation of strongly focused nanosecond laser beam inside skin tissue near and below the ablation threshold and study the collateral tissue damage above the threshold.